1. Field of the Invention
This invention relates to a gas or vapor sensor particularly one that is highly sensitive thereto.
2. The Prior Art
To protect the health of personnel, in eg. the aerospace industry, the Air Force, NASA and the like have a need to detect low concentrations of hydrazines, toxic materials used as rocket propulsion fuels. The toxicity of hydrazine (Hz) is well known; skin and respiratory exposure can yield a wide variety of harmful effects, from nausea to carcinomas. Current recommended exposure levels (threshold limit values, TLV) set by NIOSH are 30 ppb for hydrazine (Hz), 50 ppb for monomethylhydrazine (MMH) and 60 ppb for unsymmetrical dimethylhydrazine (UDMH). However, the TLVs are expected to be reduced to 10 ppb for all three compounds in the near future. For monitoring in the field, a need exists for a reliable, inexpensive, lightweight and compact sensor capable of detecting 10 ppb Hz, MMH, and UDMH with a response time of 10 minutes or less. Also, there is a need to monitor/control other alkaline reactive gases or vapors such as ammonia in a variety of applications including aquatic, fowl and vegetable farming.
In the prior art, are gas sensor units that employ two or three electrodes and liquid electrolyte in a housing. The electrolyte is usually an alkali and can escape from its container with caustic or corrosive consequences. For examples of the prior art see U.S. Pat. No. 4,839,020 to Yamaguchi et al (1989) and U.S. Pat. No. 4,666,565 to Dobson (1987).
In other prior art U.S. Pat. No. 4,820,386 to LaConti et al (1989), discloses a cation-exchange membrane with electrodes bonded thereto in a sensor. However such sensor cannot detect certain gases or vapors that are electrochemically alkaline reactive such as hydrazine type gasses and NH.sub.3. Also such sensor cell is limited to a Pt or carbon sensing electrode.
Accordingly there is a need and market for a sensor for detection of low concentrations of gas or vapors that overcomes the above prior art shortcomings.
There has now been discovered a gas or vapor sensor that avoids the use of alkaline liquid electrolyte that can leak from a container and corrode its surroundings.
Thus such sensor is durable and can detect low concentrations of gases or vapors including alkaline reactive gases as discussed below.